The present invention relates to a control system for adjusting a physical quantity of any device to a set point provided by a set-point generator, such as the temperature of an electric heating device, the speed of an electric motor or the resonant frequency of a resonant circuit, using an electronically variable adjusting element to which an analog adjusting signal is applied.
Such an adjusting element is, for example, an electronically variable resistor with which the electric current through a furnace or that of an electric motor is controlled. In the case of a resonant circuit, an element suitable for adjusting the resonant frequency is a varactor diode, whose capacitance varies with the applied voltage, as is well known.